1. Field of the Invention
The invention relates to ceramic dielectric materials and somewhat more particularly to ceramic dielectric materials which have a small temperature coefficient or permittivity, a process of producing such a material and an electrical element formed therefrom.
2. Prior Art
During the production of monolithic ceramic multi-layer capacitors, noble metals of the platinum group are typically used to form the inner electrodes thereof because of the high sintering temperatures and oxidizing atmosphere encountered during the production of such capacitor elements. Because the noble metals are extremely expensive (palladium being the least expensive metal within this group so that it is, from this point of view, a preferred metal for forming inner electrodes or at least for coating the surfaces thereof), workers in the art have made attempts to decrease the amount of such metal required, either by decreasing the electrode surface necessary for certain capacitors or by reducing the number of dielectric layers. A decrease in the amount of noble metal within a ceramic capacitor in either of the above suggested methods may be achieved by either increasing the permittivity value of the ceramic material or by reducing the dielectric layer thickness. A reducing in the thickness of a dielectric layer presupposes a higher electrical stress of the ceramic material, since at identical operating voltages, a higher field strength is present in thinner dielectric layers. Accordingly, ceramic materials having the lowest possible loss factor, the lowest possible temperature coefficient of permittivity and the highest possible insulation resistance are desired, provided that the permittivity value thereof is not overly low.
Known ceramic materials formulated on a BaO--TiO.sub.2 --ZrO.sub.2 basis have a relatively small temperature coefficient of permittivity and, unfortunately, have a relatively low permittivity value, .epsilon., of about 35. Further, such known ceramic materials are sensitive to high electrical field strengths, especially at higher operating temperatures which apparently cause an ion migration process to occur. Over a period of time, ion migrations lead to a gradual reduction of the dielectric resistance of such material.
Ceramic materials which have a better permittivity value (.epsilon. up to 80) are also known, however, these materials always include bismuth. Since palladium is not compatible with bismuth when such ceramic materials are used to form dielectric layers of a capacitor, the electrode layer must be at least partially formed of a more expensive noble metal.
German Pat. No. 967,609 suggests electric insulating bodies and capacitor dielectric layers formed of a ternary system consisting of titanium dioxide, calcium oxide and lanthanum oxide. While this material has a relatively high permittivity value, it also has a high temperature coefficient of permittivity and a relatively high loss factor.
German Auslegeschrift No. 1,005,434 suggests certain ceramic dielectric materials and capacitor dielectric layers made therefrom which consists of aluminates of bismuth, of yttrium or of an element in the lanthanide series and may include further oxide additives. However, since this material includes bismuth, it is not compatible with palladium and thus cannot be used to form capacitor dielectric layers where the inner layers are to be at least partially formed of palladium.
U.S. Pat. No. 3,400,001 suggests a ceramic dielectric material based on calcium titanate, magnesium titanate, lanthanum oxide and neodymium oxide combined with certain bismuth additives. Accordingly, this material is also not suitable for use with palladium.
On the other hand, U.S. Pat. No. 3,431,124 or, respectively, U.S. Pat. No. 3,440,067, suggest ceramic dielectric materials based on calcium titanate, lanthanum titanate and either magnesium titanate or, respectively, strontium titanate. While neither of these materials include a bismuth additive, they are unsuitable for capacitor use since they exhibit relatively high temperature coefficients of their respective permittivities.